Cracking processes such as steam-cracking, visbreaking, coking, catalytic cracking, provide C.sub.5 cuts of high olefin content. Some of them may contain substantial amounts of methyl-butenes (isopentenes). In particular, catalytic cracking C.sub.5 cuts may contain up to 30% of a mixture of 2-methyl-1-butene, 2-methyl-2-butene and 3-methyl-1-butene. An example of composition of such a cut is given in the following table:
TABLE 1 ______________________________________ BOILING HYDROCARBON % BY WEIGHT POINT .degree.C. ______________________________________ Isopentane 38.8 n-pentane 14.2 Cyclopentane 2.1 1-pentene 1.9 29.20 cis-2-pentene 6.0 37.80 trans-2-pentene 8.7 36.25 2-methyl-1-butene 9.5 38.60 2-methyl-2-butene 17.5 38.40 3-methyl-1-butene 1.0 20.00 Isoprene 0.3 ______________________________________
This table indicates the boiling points of the various olefins of the cut. It can be seen that the latter are very close to each other so that, in particular, it is impossible to separate methyl-butenes from 2-pentenes by fractional distillation.
The invention has as an object the provision of a process for the conversion of 2-methyl-1-butene continued in a C.sub.5 olefins cut, optionally containing paraffins, to 2-methyl-2-butenes. Said conversion is without substantial hydrogenation of said methyl-butenes but with substantial hydrogenation, preferably almost complete hydrogenation, of n-pentenes.
When the charge further contains a C.sub.5 diolefin, the invention has as an object the hydrogenation of said diolefin.
2-methyl-2-butene is an important raw material for many syntheses, for example for producing alcohols or ethers or producing isoprene by dehydrogenation. In these applications, the presence of C.sub.5 saturated hydrocarbons is usually not inconvenient.
In order to obtain 2-methyl-2-butene with a good degree of purity, it was necessary, up to now, to employ sulfuric acid extraction, providing an iso-pentenes mixture of 95-97% purity. During the extraction, the double bond shifts so that the final mixture contains approximately 90% of 2-methyl-2-butene and 10% of 2-methyl-1-butene. The process using sulfuric acid suffers however from many disadvantages. The first one is its high cost, mainly as a result of the sulfuric acid consumption. As a matter of fact, the presence in the charge of such diolefins as isoprene results in an overconsumption of sulfuric acid due to the formation of acid muds whose removal is always difficult and costly. A second disadvantage results from the fact that the obtained isopentene cut is not completely pure and still contains a small amount of linear olefins.